Intrinsic and extrinsic relaxation mechanisms for controlling spin current intensity in Fe100-xCox/Ta bilayers

Controlling the damping parameter in metallic ferromagnetic thin films is a key step for spintronic applications in which spin currents are generated by spin pumping. The coexistence of two states with low and high damping constant values would allow to obtain states of high and low spin current intensity, respectively. We have fabricated FeCo_x/Ta (with nominal x = 0, 15, 20, 25, 30 and 35) bilayers in which the FeCo_x layers grow epitaxially and the Ta layer is polycrystalline. We have found the coexistence of Gilbert damping and two magnon scattering mechanisms linked to a sign change in the magnetocrystalline anisotropy constant that allows the manipulation of low and high intensity states of the measured inverse spin Hall effect voltage. Bilayers with lower Co concentrations ( 25%) present different relaxation mechanisms (isotropic Gilbert damping and two magnon scattering) and an extra ferromagnetic resonance linewidth broadening produced due to mosaicity. Bilayers with Co concentration 25% present a dominating Gilbert damping for all directions in the film plane. However, in this concentration range the damping constant is anisotropic and when the magnetic field is applied along the hard magnetization direction α increases ∼420% with respect to the value obtained for the easy magnetization direction. Coexistence of isotropic Gilbert damping and two magnon scattering generated spin currents 2.5 times larger when the field is applied along the hard magnetization axis compared to the value observed in the easy magnetization axis. Thesefindings make the FeCo_x/Ta system an excellent candidate for spintronic device applications.